1. Field of the Invention
The present invention relates to a technique for fixing a toner image with heat onto a recording material.
2. Description of the Related Art
A fixing device of an electromagnetic induction heating system includes an endless heat generation member made of a magnetic metal, a coil positioned to face the heat generation member, and a pressing roller that is rotationally driven while being in press-contact with the heat generation member. Hereinafter, a region configured with the pressing roller which is in press-contact with the heat generation member will be referred to as a fixing nip portion. The fixing device applies electric current to the coil to create eddy currents in a surface of the heat generation member facing the coil, whereby the heat generation member generates heat due to Joule heat. When a recording material carrying an unfixed toner image is passed through the fixing nip portion while the fixing device controls the temperature of the heat generation member to maintain the temperature of the fixing nip portion at a predetermined temperature, the unfixed toner image is melted and fixed onto the recording material by the heat and pressure of the fixing nip portion.
When a plurality of recording materials is continuously passed through the fixing nip portion to fix toner images onto the recording materials, if the temperature of a sheet-passing region of the fixing nip portion that comes into contact with the recording materials is maintained at the predetermined temperature, the temperature of a non-sheet-passing region of the fixing nip portion that does not come into contact with the recording materials will increase excessively. This phenomenon occurs because heat in the sheet-passing region is likely to be conducted to a recording material passing through the sheet-passing region, whereas heat accumulated in the non-sheet-passing region is less likely to be conducted to a recording material passing through the fixing nip portion.
In view of the foregoing, a fixing device discussed in Japanese Patent Application Laid-Open No. 2005-55742 includes a shielding member configured to lower the magnetic flux density of a magnetic field created by the coil, and the shielding member is moved between a heat generation member and a coil to increase or decrease a shielding region. In the fixing device, the shielding member is moved to a position corresponding to the width of a recording material to decrease the magnetic flux density of a non-sheet-passing region, so that an excessive increase in the temperature of the non-sheet-passing region of the fixing nip portion is prevented.
The following describes a case in which the fixing device discussed in Japanese Patent Application Laid-Open No. 2005-55742 fixes a first toner image onto a first recording material having a first width and thereafter fixes a second toner image onto a second recording material having a second width that is wider than the first width. As used herein, the term “first sheet-passing region” refers to a region of the fixing nip portion that comes into contact with the first recording material. As used herein, the term “second sheet-passing region” refers to a region of the fixing nip portion that comes into contact with the second recording material. In the fixing device, when the shielding member is moved from a first position corresponding to the first sheet-passing region to a second position corresponding to the second sheet-passing region, the temperature of the entire second sheet-passing region does not reach a predetermined temperature at once. In other words, immediately after the shielding member is moved from the first position to the second position, a common region where the first sheet-passing region and the second sheet-passing region overlap each other is already at the predetermined temperature, whereas the temperature of a non-common region of the second sheet-passing region other than the common region remains lower than the predetermined temperature. Furthermore, after the shielding member is moved from the first position to the second position, the fixing device needs to adjust the amount of heat generation of the heat generation member such that the temperature of the common region does not reach or exceed an abnormal temperature that is higher than the predetermined temperature and the temperature of the non-common region reaches the predetermined temperature. Hence, the fixing device discussed in Japanese Patent Application Laid-Open No. 2005-55742 has a problem that it takes time for the temperature of the entire second sheet-passing region to reach the predetermined temperature after the shielding member is moved from the first position to the second position.